FINAL INDEPENDENT PROJECT

MBD analysis of a 6 dof industrial robotic arm

Abstract

In recent years, the use of robotic systems in manufacturing industries has increased tremendously. Manufacturing industries are not just looking for dumb machines which follow human instructions but smart machines which make their work flow easier. This project presents the design and dynamic behaviour analysis of an industrial robotic arm typically used for picks and place operation using Solid works 2020. The angular velocity of all the robotic links, the motor torques for all the rotary motors and the contact forces between the gripper and the object are calculated through motion studies.

Objective

1.To design the mechanism for the robot as per the requirement.

2.Create 3D models of the parts and assemble them.

3.Run motion analysis tests and interpret the results.

4.Look for possible design changes through the results to improve the efficiency of the system

Theory

Multi body dynamics

Multi body system is the study of the dynamic behaviour of interconnected rigid or flexible bodies, each of which may undergo large translational and rotational displacements.

Degrees of freedom

It refers to the number of ways a body can move in 3 dimensional space. There are two possible type of motion that is translational and rotational. The translational can be broken down into Up-down motion, Forward-backward motion and Left-right motion. The rotational type can be broken down into roll, pitch and yaw

Robotic arm

A robotic arm is a type of mechanical arm, usually programmable with similar functions to a human arm. The most commonly used industrial robot configurations are articulated robots, SCARA robots, delta robots and Cartesian coordinate robots.

Applications of robotic arm

1.Welding

2.Material handling processes like pick and place, packaging, machine loading etc.

3.Mechanical cutting, grinding, polishing and painting.

4.Gluing, adhesive sealing and spraying materials.

(KUKA robot assembling a car, Photo courtesy: Daimler AG)

Part Modelling

Base

The base is the hard rigid component on which the entire robotic arm setup is placed.

Waist part

The waist part stays right over the base and it can rotate around its axis.

Shoulder part

The shoulder is mounted on the waist and provides forward and backward motion

Elbow part

The elbow is fixed onto the smaller end of the shoulder and it provides up and down motion

Gripper support

The gripper support connects the gripper and the elbow. It provides rotational motion to the wrist part of the robotic arm.

Gripper holder

The gripper holder acts as a bridge between the gripper support the wrist part. It provides the up and down motion (pitch) to the gripper.

Wrist part

The wrist part provides rotational motion to the manipulator (roll)

Gripper

The gripper (manipulator) is the part which acts as fingers or claw to pick and place an object.

Robotic arm assembly

The parts were assembled through bottom-up approach (base to gripper) by giving proper mate conditions keeping in mind the required degrees of freedom.

Technical Description

Material

Robot arms: Alloy steel

Manipulator: Cast alloy steel

Property	Alloy steel	Cast alloy steel	Unit
Elastic modulus	0.21E+12	0.19E+12	N/m2
Poisson’s ratio	0.28	0.26	-
Shear modulus	0.79E+11	0.78E+11 N/m^2	N/m2
Thermal expansion coefficient	0.13E-4	0.15E-4	/K
Mass density	0.77E+04	7300	Kg/m3
Thermal conductivity	50	38	W/(m-k)
Specific heat	0.46E+03	440	J/(kg-K)
Tensile strength	7.238256E+8	4.480825E+8	N/m2
Yield strength	6.20422E+8	2.412752E+8	N/m2

Degrees of freedom

According to grubler’s criterion,

F=3(n-1)-2j-h

F=3(7-1)-2(6)-0

F=6

Where,

F=No of degrees of freedom

n=No of links

j=No of joints

h=No of higher pairs

Robot workspace

It is the region of space the robot can reach. It depends on the number of axes, degrees of freedom, angle and translational limitations. The base of this robotic arm can rotate at 360 degrees. The shoulder joint can move between 30-180 degrees. The elbow joint can move from 50-180 degrees. The gripper support can rotate at 360 degrees. The gripper holder (pitch motion) can move from 0-90 degrees. The wrist part (roll motion) can rotate at 360 degrees and both the grippers can move from 0-90 degrees. The maximum reach of the robot is around 785mm.

Animation representing the motion of the robotic arm

The setup

The assembled robotic arm was placed onto a floor which has two wooden cube structures named as Point A and Point B. The centre of the robotic arm is at distance of 359.78mm (approx 360mm) from point ‘A’ and 340.22 mm (approx 340mm) from point ‘B’. The positions and motion of each and every link of the robotic arm was controlled by the mate controller.

Animation representing the working of the robot

Motion analysis plots

The robotic arm reaches point A picks up the work piece, places it at point B and then comes back to its initial position

Motor torque vs time

Motor torque of waist joint

Motor torque of shoulder joint

Motor torque of elbow joint

Motor torque for wrist joint (roll motion)

Angular velocity vs time

Angular velocity of waist joint

Angular velocity of shoulder joint

Angular velocity of elbow joint

Angular velocity of the wrist joint

Angular velocity of the gripper (left)

Angular velocity of the gripper (right)

Contact forces vs time

Contact force between left gripper and the object

Contact force between right gripper and the object

Observations

1. The motor torque in all the cases is relatively small as the working area of the robot is very small.
2. The motor at the shoulder joint and the motor at the waist has the highest motor torque and hence the power consumed by those motors are the highest.
3. The motor torque value of the wrist motor is very low as the wrist rotational motion is very slow and simple.
4. All the angular velocity graphs have 3 major parts. In the first part the robot moves from its initial position to point A. In second part it lifts the part and moves from point A to point B. In the third part it returns from point B to initial position. The parts in the middle indicate the slight motion of the arm in order to adjust itself so that it can place the object at the exact position.
5. The maximum angular velocities of the waist, shoulder and elbow joints are 48 deg/s as they all coordinate together to pick and place the object precisely
6. The angular velocity of the waist joint from 0-2.01s is around 25.00 deg/s but the value is quiet less at the end that is 20 deg/s at the time limit 25.01-27s. This is because the distance between the robot and point A is greater than that of the distance between the robot and point B
7. The angular velocity of both the grippers is almost the same as they move exactly at same distance and angles to pick and place the object.
8. The contact forces between the object and the grippers are very low and they have lots of jerks as the object slides over the upper surface of the gripper. This can be avoided if the gripper directly holds the body of the object to lift it.

Practical use of the study

1. In design scenario, the motor torque value could be used to choose the correct sized motor (optimal) and thus reducing the cost and time.
2. The contact forces between the gripper and the object will help in deciding the choice of the material to be used for gripper as per its working environment.
3. The displacement and velocity values can be used in LabVIEW and this will help in the development of virtual prototyping.
4. The velocity values of the robot will help at designing the robot for a particular operation. (Example:For huge automobile assembly operations, the speed has to be high) The velocity values can be adjusted by properly understanding the requirement and calculating its working envelope (workspace)
5. For the further development of the robot (electronics and coding part) these kinematic data will come handy to produce accurate results.

Future scope

1. The power consumption of each motor can be calculated which help in choosing the exact motor.
2. The frictional forces between the parts and between the gripper and the object can be studied.
3. Grippers with inbuilt temperature sensors (known as active grippers) can be used. This can be widely used in pharmaceutical and chemical industries where the product has to be produced at a particualar temperature. The robotic arm will help in sorting the defective (not in optimal temperature) and good products.

Rendered Images

References

1. MBD using solidworks course, skill lync, accessed on June 2020
2. An Introduction to Motion Analysis Applications with SolidWorks Motion, Instructor Guide
3. Virendra Patidar,Ritu Tiwari,Survey of Robotic Arm and Parameters, International Conference on Computer Communication and Informatics (ICCCI -2016)
4. Rosidah Sam, Kamarul Arrifin,Norlida Buniyamin, Simulation of Pick and Place Robotics System Using Solidworks Soft motion, 2012 International Conference on System Engineering and Technology